1. Technical Field
The present disclosure relates to an image processing apparatus and an image processing method. More particularly, the present disclosure relates to an image processing apparatus and an image processing method, which perform image processing in response to an operation of changing the display angle or the display part of an image visualizing tubular tissues.
2. Description of the Related Art
Computed Tomography (CT) and Magnetic Resonance Imaging (MRI), which make it possible to directly observe the internal structure of a human body, have brought about an innovation in the medical field according to the image processing technology using a computer, and medical diagnosis using the tomographic image of a living body has been widely conducted. Further, volume rendering has been used for medical diagnosis in recent years. The volume rendering enables to visualize the complicated three-dimensional structure of the inside of a human body, which is hard to understand simply from the tomographic image of the human body. For example, the volume rendering enables to directly render an image of the three-dimensional structure from three-dimensional digital data (volume data) of an object obtained by CT.
In the volume rendering, a virtual ray is applied to volume data (three-dimensional voxel space), whereby an image is projected onto a projection plane. A ray cast method is available as a kind of this operation. In the ray cast method, sampling is performed at given intervals along the ray path and the voxel value is acquired from the voxel at each sampling point. Color information and opacity are calculated from the voxel value. The voxel is a unit of a three-dimensional region of an object and the voxel value is unique and representing the characteristic of the voxel such as the density value of the voxel. The volume data are represented by a three-dimensional array of the voxels.
The ray cast method, a Maximum Intensity Projection (MIP) method, a Minimum Intensity Projection (MinIP) method, a Multi Planar Reconstruction (MPR), a Curved Planer Reconstruction (CPR) and the like are used as three-dimensional image processing in the volume rendering. Also, a 2D slice image is generally used as two-dimensional image processing.
FIG. 9 is a drawing to show an example of a cross section cut out from volume data when the MPR is used as three-dimensional image processing in the volume rendering. As shown in FIG. 9, according to the MPR, a certain cross section 11 can be cut out from a three-dimensional volume data 51 and can be displayed. FIG. 10 is an image showing the internal tissue of a human body provided by using the MPR.
FIG. 11 is a drawing to show an example of a cross-sectional curved surface along a certain path, which is cut out from volume data, when CPR (Curved MPR) is used. As shown in FIG. 11, according to the CPR, certain cross-sectional curved surfaces 52, 53, 54, 55, and 56 along a certain path 57 in the three-dimensional volume data 51 can be cut out. Further, a continuous image 50 of the cross sections can be displayed as a planar image. Accordingly, the CPR is suited to image representation of a winding organ such as a blood vessel or an intestine. For example, the image of a blood vessel 61 in volume data shown in FIG. 12 is generated based on the voxel values of the voxels on a cross-sectional curved surface 63 along the center line of the blood vessel 61. FIG. 13 is an image showing the internal tissue of a human body provided by using the CPR.
The curved surface determined by the CPR can be defined as shown in FIG. 14. Namely, the curved surface S shown in FIG. 14 is defined by a set of plural lines li with uniform direction vector (hereinafter referred to as “CPR direction vector”) v passing through certain points on the curve c in volume data. A processing apparatus such as a computer displays a two-dimensional image, based on the values of the voxels on the curved surface S (hereinafter referred to as “CPR image”).
In addition, the CPR direction vector v, which is one of the parameters defining the curved surface S, can be set in any desired direction. If the CPR direction vector v is changed, an organ (e.g., a blood vessel or an intestine) represented by the curve c can be observed in a multifaceted manner. Therefore, when the user changes a CPR direction vector, the processing apparatus determines a curved surface corresponding to the CPR direction vector and displays a CPR image on the determined curved surface on the display.
As the operation of changing the CPR direction vector, for example, the operation of dragging a pointer in any desired direction on the CPR image is done. If the user drags the pointer up-and-down on the CPR image as shown in FIG. 15A, the CPR direction vector is rotated about the axis orthogonal to the CPR direction vector in volume data as shown in FIG. 15B, and the processing apparatus determines a curved surface corresponding to the CPR direction vector thus rotated. If the user drags a pointer up-and-down and from side to side on the CPR image as shown in FIG. 16A, the CPR direction vector is changed in any desired direction in volume data as shown in FIG. 16B, and the processing apparatus determines a curved surface corresponding to the CPR direction vector thus changed. (see e.g., “CPR—Curved Planar Reformation” written by Armin Kanitsar et al.)
However, if the user drags the pointer up and down on the CPR image of the curved surface such that the tangent vector of the curve c is substantially equal to the CPR direction vector, there is a possibility that the CPR image may be distorted. Further, two degrees of freedom of rotation are involved in the CPR direction vector while the operation of dragging the pointer up and down is limited to only one degree of freedom as shown in FIGS. 15A and 15B. Thus, the desired CPR image may not be obtained.
As shown in FIGS. 16A and 16B, if the user can perform the operation of changing a CPR direction vector in any desired direction in a three-dimensional space in response to the operation of dragging not only up-and-down but also from side to side, this dragging operation is performed on a two-dimensional image (CPR image) and further change in the CPR direction vector responsive to this operation is made in a three-dimensional space. Therefore, it is very difficult for the user to imagine the curved surface defined by a set of plural lines of CPR direction vector after rotation passing through certain points on the same curve as before the change. Consequently, in order to obtain any desired CPR image, the user needs to repeat rotating the CPR direction vector by trial and error. Particularly, when the CPR direction vector is rotated, it is likely that the user has the false impression that the curved surface S is rotated such that the shape is maintained.
Further, the user requires a high operation capability in cases where the user drags the pointer up-and-down and from side to side repeatedly by much trial and error to restore the CPR direction vector to the former state. This is because generally commutation rules do not hold in multiplication of a matrix involved in rotation operation.